loud ringer

They have devices that act as a voip endpoint (note - just did a quick google search 'voip external speaker' for the link -https://www.bogen-paging.com/articles/485.aspx) This would need a network line ran to it to the garage.

These can be set up manually in most cases (GenSIP) but if you find one that looks good for your situation, I would check with RC for compatibility. (they may have a list as well)

Some - you can have set as both a 'ring'er and a loudpeaker for an intercom/annoucements.

Hope that helps.

You can get a SNOM PA1, register it as a device on RingCentral and have it ring simultaneously with your shop phone.

You can connect an 8-ohm loud speaker to it and you're good to go.